A quantum-well structure operated in accordance with the quantum confined Stark effect near the edge of its absorptive band exhibits the largest optical nonlinearity measured to date in any semiconductor at room temperature. This characteristic makes such quantum-well structures of particular interest in the fabrication of a variety of electro-optic devices, including, for example, optical modulators, photodetectors, optical switches, and the like. For a general discussion of quantum-well electro-optic devices, the reader is directed to the article: "Quantum Wells For Photonics", by D. S. Chemla, Physics Today, May, 1985, pgs, 57-64. For a more thorough discussion of the quantum confined stark effect, the reader is directed to the article: "Electric Field Dependence of Optical Absorption Near the Band Gap of Quantum-Well Structures", by D. A. B. Miller, et al., Physical Review, Vol. 32, No. 2, 15 July 1985, pgs 1043-1060.
For a discussion of the use of quantum-well devices to modulate long wavelength light, the reader is directed to the article "Long-Wavelength Optical Modulation in Multiple Quantum Wells", by K. Wakita et al., Surface Science 174 (1986), pgs 233-237.
Quantum-well electro-optic devices are used today in two general modes. A first mode is that wherein incident light is directed generally parallel to the plane of the multiple layer heterostructure, i.e. the "edge-on" mode. For a discussion of such quantum-well devices, the reader is directed to the article: "Strong Polarization-Sensitive Electroabsorption in GaAs/AlGaAs Quantum Well Waveguides", by J. S. Weiner, et al., Applied Physics Letter, Vol. 47, No. 11, 1 Dec. 1985, pgs. 1148-1150. A second mode is that wherein the incident light is directed generally perpendicular to the plane of the heterostructure.
U.S. Pat. No. 4,525,687 to Chemla et al. provides a complete discussion of quantum well devices, including devices operated edge on, and devices operated with perpendicular incident light. Chemla et al. shows devices operated with electrical fields both parallel and perpendicular to the planes of the devices. Chemla et al. further discusses the use of such devices as optical absorption modulators, optical phase modulators, electrically tuned Fabry-Perot cavities, polarization modulators, and switches.
U.S. Pat. No. 4,620,214 to Margalit et al. shows a device including an infrared detector comprising multiple, alternating layers of GaAs and Ga.sub.1-x Al.sub.x As. The structure shown in Margalit et al. is an "edge on" detector, wherein light is directed incident to the edges of the heterostructure.
Published European Patent Application EPA No. 0 249 645, assigned to the assignee of the present invention, shows an optoelectronic voltage-controlled modulator including a quantum well structure functioning as an optical absorber, and a superlattice functioning as a buried reflector, both formed on a GaAs substrate. A light beam is applied relatively normal to the heterolayer device so as to reflect through the quantum well structure from the buried reflector. A control voltage is applied across the device to determine the degree of absorption effected by the optical absorber, and hence to modulate the reflected light.
U.S. Pat. No. 4,218,143 to Bottka shows a semiconductor device incorporating multilayer, mismatched lattice structures used to measure the wavelength of incident light. The Bottka device includes a structure incorporating multiple layers of GaAs.sub.1-s Sb.sub.x. Incident monochromatic light is reflected through the semiconductor device off of a Schottky barrier contact. A modulated electric field is applied to effect modulation of, and hence aid in detecting, light absorbed at the energy gap(s) in the lattice structure. In Bottka, the incident light is directed generally normal to the multilayer structure.
IBM Technical Disclosure Bulletin, Vol. 8, No. 11, April 1966, pgs. 1557-1559, "Electrophotographic Light Amplifier", shows a light amplifier comprising a multilayer structure including adjoining layers of photoconductive material and Stark modulator material.
"Active Q Switching in a GaAs/AlGaAs Multiquantum Well Laser with an Intracavity Monolithic Loss Modulator", by Y. Arkawa et al., Applied Physics Letter, Vol. 48, No. 9, 3 Mar. 1986, pgs 561-563, shows a multiquantum laser including a modulator section exhibiting the quantum confined Stark effect.
When devices employing multiple quantum well heterostructures of the type described above are used to modulate incident light, several disadvantages become apparent. More specifically, devices of the type designed to modulate normally (perpendicular) incident light require a substantial number of quantum wells in order to effect an acceptably high absorption of the incident light. This is true even in reflective devices, such as that shown in EPA No. 0 249 645, wherein the absorptive layer is traversed twice: once by the incident light and once by the reflected light.
In such devices, it is thus required to grow the heterostructure to a substantial thickness, i.e. to incorporate many quantum wells into the heterostructure. The resultantly thick device requires an undesirably large electrical field to effect a Stark shift, requiring the use of an undesirably high electrical potential to bias the device.
In devices designed to modulate parallel incident light, i.e. edge on devices, the wavelength of the light is typically much longer than the thickness of the quantum well. It is thus required to grow an undesirably thick heterostructure (as described above), or to go to substantial efforts to couple the incident light into a thin heterostructure. Such efforts can include, for example, the formation of optical waveguides on the surfaces of the heterostructure.
It is known in the art that a localized electromagnetic field can be established by launching a surface polariton at a semiconductor interface with a discontinuity from a positive to a negative dielectric constant. This process can be used to concentrate a light source incident to the interface into a region with a locally high energy density. For a further discussion of this phenomenon, the reader is directed to the article: "The Electromagnetic Modes of Media", by D. L. Mills, et al. Rep. Prog. Phys. 37, 817 (1974). Along the same lines, the reader is further directed to the article: "Photon Emission From Slightly Roughened Tunnel Junctions", by B. Laks et al., Physical Review, Vol. 20, Number 12, 15 Dec. 1979, pgs. 4962-4980. For a discussion of the same phenomenon with the use of gratings, the reader is directed to the article: "Diffraction-Grating-Enhanced Light Emission From Tunnel Junctions", by Kirtley et al., Applied Physics Letter, Vol. 37, Number 5, 1 Sept. 1980, pgs. 435-437.
U.S. Pat. No. 4,025,939 to Aiki et al. shows a semiconductor laser device employing a quantum well structure with the laser active region comprising the narrow bandgap region. A periodically corrugated surface is employed at the boundary of an optical confinement region to diffract the light and hence control the lasing frequency. Because the corrugated surface in Aiki et al. does not provide the necessary discontinuity in the dielectric constants, no surface polaritons are launched.
2. Summary of the Invention
The principal object of the present invention is to provide a new and improved method and apparatus for modulating light energy.
Another object of the present invention is to provide such a method and apparatus which effects a large change in the intensity of normally incident light using a thin electro-optical structure.
A further object of the present invention is to provide such a method and apparatus which does not require the high electrical voltages or complex incident light coupling of the prior art.
In accordance with the present invention, a new and improved method of modulating light incident to a semiconductor body is provided, comprising the steps of: coupling the incident light to the surface plasmon polariton mode at an interface of the semiconductor body; and selectively altering the absorption of the incident light by the semiconductor body so as to decouple the incident light from the surface plasmon polariton mode.
In one embodiment of the invention, the step of altering the absorption of the incident light includes the steps of: establishing a quantum confined optical absorption region within the semiconductor body; and effecting a Stark shift of the quantum confined optical absorption region.
Further in accordance with the present invention, apparatus is provided for modulating light incident to a semiconductor body, comprising: means for coupling the incident light to the surface plasmon polariton mode at an interface of the semiconductor body; and, means for selectively altering the absorption of the incident light by the semiconductor body so as to decouple the incident light from the surface plasmon polariton mode.
In one embodiment of the invention, the means for altering the absorption of the incident light includes: means for establishing a quantum confined optical absorption region within the semiconductor body; and means for effecting a Stark shift of the quantum confined optical absorption region.